An EDM process of this type has commonly been carried out in the past in a dielectric fluid such as kerosene (paraffin) or transformer oil retained in a worktank. A workpiece is deeply immersed in the dielectric fluid in the worktank and a tool electrode is also dipped into the dielectric fluid and spacedly juxtaposed with the workpiece. The machining gap defined between the tool electrode and the workpiece is thus deeply immersed in the dielectric fluid in the worktank. A succession of electrical discharges are effected between the tool electrode and the workpiece across the machining gap to electroerosively remove stock from the workpiece. As stock removal proceeds, the tool electrode is advanced into the workpiece until the required depth of a desired cavity is reached.
While the conventional method described has been found to be generally satisfactory and acceptable for commercial purposes, there is consistent desire to increase removal rate and in general, machining efficiency. In addition, it should be noted that the dielectric fluid of the type described is highly flammable so that a number of measures are required to prevent a fire and to fight a fire if it happens to occur. On the other hand, in wire-cut EDM, a type of EDM quite different in machining mode from cavity-sinking EDM, it has been the common practice to employ as the machining fluid pure or distilled water which is not inflammable at all. Thus, attempts have been made to employ the pure or distilled water machining fluid in the cavity-sinking EDM process. It has proved, however, that the cavity-sinking efficiency obtained with the water fluid is much inferior to that obtainable with kerosene or transformer oil.